1. Field of the Invention
The present invention relates to an apparatus and a method for monitoring an I-Q phase bias in an I-Q quadrature modulation transmitter. The method is suitable for a variety of communication systems, optical communication systems or other communication systems, that use in-phase branching and quadrature-phase branching, such as a Differential Quadrature Phase-Shift Key (D QPSK), a Quadrature Phase-Shift Key (QPSK), a Multi Phase-Shift Key (M-PSK), and a Differential Multi Phase-Shift Key (DM-PSK) Quadrature Amplitude Modulation (QAM).
2. Description of the Related Art
The data transfer capacity of optical communication has increased dramatically in the past several years. The traditional binary amplitude shift keying (also known as on-off key, OOK) of non-zero-return (NRZ) or zero-return (RZ) still dominates as the practical modulation technology. Recently, many new modulation and demodulation technologies, such as duobinary, Carrier Suppressed Return-to-Zero (CSRZ), and differential phase-shift keying (DPSK), have been introduced in optical communications. In the DPSK modulation, information is represented by a phase change of two neighboring symbols. In the binary DPSK, phase change is 0 or π. If phase changes are 0, π/2, π or 3π/2, the modulation technology is called Differential Quadrature Phase-Shift Keying (DQPSK). Compared with traditional OOK technology, phase-shift keying has advantages in terms of an optical signal-to-noise ratio (OSNR) gain of 3 dB, and a strong anti-non-linearity capability. By using the quaternary symbols, optical DQPSK doubles the utilization of the frequency spectrum, while the requirements for the electric device speed, the optical dispersion management, and the polarization mode dispersion are lowered. It is expected that optical DQPSK will play an important role in the next generation of optical communications.
According to the article Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission (R. A. Griffin et al., OFC 2002), the contents of which are hereby incorporated by reference, a typical optical DQPSK transmitter consists of: a splitter to divide an input optical signal into an I branch signal and a Q branch signal; an I-branch modulator (1 or −1) and a Q-branch modulator (1 or −1), with a phase bias on the Q-branch; and a combiner to combine the I-branch signal and the Q-branch signal into a modulated signal. In order to ensure the quadrature of the I-branch and the Q-branch, the phase bias should be π/2. A phase bias different from π/2 would cause an additional optical signal to noise ratio (OSNR) penalty. A feedback control is usually adopted, in which a monitoring device monitors a phase error of the phase bias and generates a phase error signal to adjust the phase bias, so as to lock the phase bias to the π/2 value. An often-used feedback control technology is the dither-peak detection. A typical configuration of a dither-peak detection scheme is displayed in FIG. 1. A phase bias generator 108 dithers at a fixed frequency f, while a monitoring device 002 outputs a corresponding phase error monitoring signal. When the phase is at the target value of dither (as illustrated in 005), the error signal reaches a maximum or a minimum value. The control logic device 004 adjusts the DC bias 003 according to whether the monitoring signal has reached a peak value (the maximum or the minimum value), so that the phase bias 108 is set to an optimal point. The traditional dither-peak detection scheme (as exemplary illustrated in FIG. 1) has the following intrinsic defects:
1. The phase dithering will lead to an additional optical signal to noise ratio (OSNR) penalty.
2. The peak detection method can only determine whether the phase is at a target value. It cannot indicate whether the current phase is greater or smaller than the target value.
3. The monitoring signal obtained via the peak detection is in a square relation to the phase error, therefore the peak detection signal becomes far less sensitive when approaching a zero error point, which leads to a low phase control precision.
4. The speed of the phase control is constrained by the dithering frequency.
A new phase control method is needed to overcome the above-mentioned defects.